Automated drilling of holes in circuit boards is typically accomplished by mounting the circuit board to the worktable of an automated circuit board drilling machine. An example of such a machine would be the the Concept 4 drilling and routing machine available from Excellon Automation of Torrence, Calif. The circuit board is accurately positioned on a worktable platten below a spindle which rotates a drill bit. The spindle is lowered and the drill bit creates a hole through the circuit board. The entire drilling process is usually coordinated by a host processor The processor controls such things as worktable positioning, spindle feed rates, spindle rpm, and automatic drill bit changing.
The need for small pads, thin conductor lines, and narrow line spacings in printed circuit boards requires automated circuit board drilling machines to have both high positional accuracy and the ability to drill high quality, small diameter holes. Verification of drill bit diameter, runout, and tip position is necessary to ensure high hole quality.
Drill bit diameter checking is necessary to avoid drilling the wrong hole size and thus producing defective boards. Drill bit diameter has in the past been checked "off-line" by a mechanical micrometer. However, there is a risk of damaging or breaking the drill bit when using contact measurement tools, especially with very small diameter drill bits. (Typical drill bit diameters can range from 0.004 inch to 0.250 inch.) Moreover, off-line measurements of drill bit diameter lack the ability to make dynamic measurements of drill bit dimensions, and cannot make real time verifications that the proper size bit is loaded in the spindle.
Runout is defined as the amount of asymmetry in the drill position as it rotates. The asymmetric motion may be due to a bent or wobbling drill, asymmetry of the drill geometry about its center of rotation, or vibrations in the spindle. Excessive runout can result in enlarged holes, inaccurate drilling, or tool breakage. Currently, runout is typically measured quasi-statically by placing a mechanical dial indicator in contact with a precision pin inserted in the spindle. The spindle is then rotated by hand and the amount of deflection of the dial indicator is noted. Dynamic runout measurements may be made by using a capacitive sensor and a precision pin. Both of these methods, however, only measure spindle runout. Neither method measures dynamic runout of each individual drill bit under actual usage conditions. The mass of the drill bit, its geometry, its seating in the chuck, and the spindle rpm all affect the runout. Smaller diameter drills are typically operated at higher rpm's than larger diameter drills, exacerbating the discrepancy between static and dynamic runout measurements.
Accurate drill tip positioning is required to drill blind hole vias. Blind hole vias are holes which are drilled only partially through a circuit board and are used to connect separate layers of a multilayer circuit board. (See FIG. 1a.) Accurate tip positioning is also required to increase production throughput. Proper and accurate vertical positioning of the drill bit avoids the wasted time caused by drilling or retracting the drill too far, and also prevents tool breakage caused by drilling too far past the circuit board.
While noncontact equipment for measuring the dimensions of objects such as drill bits is available, conventional noncontact instruments generally require a scanning laser beam, and are typically too bulky to be used on-line in conjunction with an automated circuit board drilling machine. In addition, the measurement methodology of such previous equipment is unable to differentiate between drill diameter and runout, and often provides diameter and runout as a single, combined reading. Conventional drill tip positioning methods require contact with the drill bit, often causing chipping or breakage of the drill bit. Moreover, conventional laser based sensors placed in proximity to an automated circuit board drilling apparatus would be susceptible to measurement error due to the varying occlusion of light in the measurement field by the dust residue caused by the drilling. As such, there have not been any automated circuit board drilling machines capable of verifying drill bit diameter immediately after each individual drill is loaded in the spindle, of determining dynamic runout, or capable of accurately positioning, in a noncontacting manner, the drill tip in relationship to a board mounted for drilling.